1. Field of the Invention
This invention relates generally to organoborane polyamine complexes and, more specifically, to those complexes in which the polyamine is the reaction product of a diprimary amine-terminated material and a material having at least two groups reactive with primary amine. The invention further relates to the use of these complexes in systems for initiating the polymerization of acrylic monomers, as well as acrylic adhesive compositions made therewith. The adhesive compositions have excellent adhesion to a variety of substrates, especially low surface energy polymers.
2. Description of the Related Art
Organoboranes such as tributylborane and triethylborane have been reported to initiate and catalyze the polymerization of vinyl monomers (see, for example, G. S. Kolesnikov et al., Bull. Acad. Sci. USSR, Div. Chem. Sci. 1957, p. 653; J. Furakawa et al., Journal of Polymer Science, volume 26, issue 113, p. 234, 1957; and J. Furakawa et al., Journal of Polymer Science, volume 28, issue 116, 1958). The organoborane compounds of the type described in these references are known to be quite pyrophoric in air which complicates facile use.
Chemical Abstracts No. 134385q (volume 80, 1974) "Bonding Polyolefin or Vinyl Polymers" reports that a mixture of 10 parts methyl methacrylate, 0.2 part tributylborane, and 10 parts poly(methylmethacrylate) was used to bond polyethylene, polypropylene and poly(vinyl acetate) rods.
U.S. Pat. No. 3,275,611 to E. H. Mottus et al. discloses a process for polymerizing olefinic compounds with a catalyst comprising an organoboron compound, a peroxygen compound, and an amine. The organoboron compound and the amine may be added to the reaction mixture separately or they may be added as a preformed complex. The latter approach reportedly has the advantage of making the boron compound more easily handled, especially for certain boron compounds that tend to be pyrophoric in air but which are not pyrophoric when complexed. Especially useful boron catalysts are said to have the following general formulas: R.sub.3 B, RB(OR).sub.2, R.sub.2 B(OR), R.sub.2 BOBR.sub.2, R.sub.2 BX, and R.sub.2 BH, where R is preferably an alkyl radical having from 1 to 10 or more carbon atoms, and X is a halogen. Various amine complexing agents are mentioned although pyridine, aniline, toluidine, dimethylbenzylamine, and nicotine are used in the examples.
While Mottus et al. refer to polymerizing methacrylate monomers, there is no indication that the resulting polymers are useful as adhesives. Various acids are mentioned as monomers that may be polymerized but there is no indication that an acid is a component of the polymerization initiator system.
British Patent Specification No. 1,113,722 "Aerobically Polymerisable Compositions," published May 15, 1968 discloses the polymerization of acrylate monomers through the use of a free-radical catalyst (e.g., peroxides) and triarylborane complexes having the general formula (R).sub.3 B-Am wherein R is an aryl radical having from 6 to 12 carbon atoms and Am is an amine that can be selected from various classes such as alkylamines, cycloalklyamines, aralklyamines, polyamines (e.g., alkylene diamines and triamines), and heterocyclic amines. The polymerization is activated by heat or the addition of an acid. The resulting compositions are reportedly useful as adhesives.
Chemical Abstracts No. 88532r (volume 73, 1970) "Dental Self-curing Resin" and the full text paper to which it refers report that tributylborane can be made stable in air by complexing it with ammonia or certain amines (e.g., aniline, n-butylamine, piperidine, ethylenediamine) at a mole ratio of one and that the tributylborane can be reactivated with an amine acceptor such as an isocyanate, an acid chloride, a sulfonyl chloride, or anhydrous acetic acid. As a result, the complex can be used to polymerize blends of methyl methacrylate and poly(methylmethacrylate) to provide a dental adhesive. Tributylboraneethylenediamine complexes and triethylborane-ammonia complexes, each with p-toluenesulfonyl chloride as the amine acceptor, are specifically mentioned.
A series of patents issued to Skoultchi and Skoultchi et al. (U.S. Pat. Nos. 5,106,928, 5,143,884, 5,286,821, 5,310,835, and 5,376,746) disclose a two part initiator system that is reportedly useful in acrylic adhesive compositions, especially elastomeric acrylic adhesives. The first part of this two part system includes a stable organoborane amine complex and the second part includes a destabilizer or activator such as an organic acid or an aldehyde. The organoborane compound of the complex has the general formula: ##STR1## where R, R.sub.1 and R.sub.2 are either alkyl groups having 1 to 10 carbon atoms or phenyl groups. Useful amines include n-octylamine, 1,6-diaminohexane, diethylamine, dibutylamine, diethylenetriamine, dipropylenediamine, 1,3-propylenediamine, and 1,2-propylenediamine.
The adhesive compositions are reportedly particularly useful in structural and semi-structural applications such as speaker magnets, metal-metal bonding, (automotive) glass-metal bonding, glass-glass bonding, circuit board component bonding, selected plastic to metal, glass, wood, etc. bonding, and electric motor magnets. Those plastics that may be bonded are not further described.
An efficient, effective means for adhesively bonding low surface energy plastic substrates such as polyethylene, polypropylene and polytetrafluoroethylene (e.g., TEFLON) has long been sought. The difficulties in adhesively bonding these materials are well known. See, for example, "Adhesion Problems at Polymer Surfaces" by D. M. Brewis that appeared in Progress in Rubber and Plastic Technolgy, volume 1, page 1 (1985). The conventional approaches typically function by: (1) increasing the surface energy of the substrate (to more closely match the surface energies of the substrate and the adhesive thereby promoting better wetting of the substrate by the adhesive) and/or (2) eliminating additives and low molecular weight polymer fractions in the substrate that can migrate to the substrate surface and adversely affect adhesion by forming a weak boundary layer.
As a result, the conventional approaches often use complex and costly substrate surface preparation techniques such as flame treatment, corona discharge, plasma treatment, oxidation by ozone or oxidizing acids, and sputter etching. Alternatively, the substrate surface may be primed by coating it with a high surface energy material. However, to achieve adequate adhesion of the primer, it may be necessary to first use the surface preparation techniques described above. All of these techniques are well known, as reported in Treatise on Adhesion and Adhesives (J. D. Minford, editor, Marcel Dekker, 1991, New York, volume 7, pages 333 to 435). The known approaches are frequently customized for use with specific substrates. As a result, they may not be useful for bonding low surface energy plastic substrates generally.
Moreover, the complexity and cost of the presently known approaches do not render them particularly suitable for use by the retail consumer (e.g., home repairs, do-it-yourselfers, etc.) or in low volume operations. One vexing problem is the repair of many inexpensive everyday household articles that are made of polyethylene, polypropylene or polystyrene such as trash baskets, laundry baskets and toys.
Consequently, there has been a considerable and long felt need for a simple, easy to use adhesive that can readily bond a wide variety of substrates, especially low surface energy materials, such as polyethylene, polypropylene and polytetrafluoroethylene, without requiring complicated surface preparation, priming and the like. It would also be considered useful for the adhesive to be able to bond a wide variety of diverse surfaces, including metals.
While an adhesive that can bond low surface energy plastics is certainly advantageous, the commercial utility of such an adhesive would be enhanced if the components thereof could be combined in a convenient mix ratio. This would permit facile application of the adhesive using conventional adhesive dispensers without the need for laborious hand weighing and mixing of the different components. However, the convenient mix ratio should not come at the expense of significantly reduced storage stability or performance. Thus, there is not only a need for an adhesive that can bond low surface energy plastics, but a need for such an adhesive that can be readily blended in a convenient mix ratio without a material reduction in storage stability or performance.